i would not say it to someone balding in front, but iif their hair is perfect in front and on top, but thinning and not slick in back, i would feel very tempted to let them know, since they might not know yet and can't feel it in the show. That can get 4 inches wide in under 6 months. No one in my family told my brother. He just found it one day. I'm sure that was depressing.
But I don't want anyone getting pissed at me. Many guys have gotten pissed when i suggested propecia, and say their girlfriend said she would never leave him even if he goes bald. I'm sure every couples that to each other in the beginning. Has anyone ever been told by his girlfriend in the beginning of a relationship, or before marriage, that if he gets in a car crash or goes bald or something happens to him, she will leave him? I doubt it. But when one partner's appearance changes enough, and the other stays the same or changes the other direction, and it is obvious it will be a long lasting change, that is when you see how much those words meant. Most won't break up right away, but they will act less interested and foil the relationship one way or another to avoid saying the real reason. If fact, there may have been other flaws in someone the whole time which did not matter because she was having fun, but now that she is not having fun, she will point out all those flaws and tell him to fix them, and he will say they never bothered her before and he can't change them all right away or does not think it is fair unless she changes her faults too, and that is when she will say that maybe it just is not working out between them any more.